The present invention relates to a continuously variable transmission of the half-toroidal type for use as an automotive transmission, for example, and more particularly, to an improvement of discs.
A continuously variable transmission of the half-toroidal type, such as the one described in U.S. Pat. No. 5,372,555, is provided with an input shaft capable of rotating integrally with a drive source, such as an engine, and an input disc and an output disc supported on the input shaft. A cam flange is provided at the back of the input disc. The cam flange is in spline-engagement with the input shaft. Cam rollers are interposed between the cam flange and the input disc. The input disc is pressed toward the output disc by means of a push mechanism of the loading-cam type.
Trunnions, which are rockable around a pivot each, are arranged between the input and output discs. A displacement shaft is located in the central portion of each trunnion. A power roller is rotatably supported on the displacement shaft. The power roller has a traction portion in rolling contact with the input and output discs. Between the input and output discs, the power roller can rock around the pivot of the trunnion, thereby changing its angle of inclination, depending on the speed change ratio between the discs.
A ball thrust bearing, which serves as a power roller bearing, is provided with between each trunnion and its corresponding power roller. This bearing bears a thrust-direction load on the power roller and allows the roller to rotate. The rotation of the input disc is transmitted to the output disc through the power roller. High pressure is generated in each of the discs and the power roller. This pressure can be explained according to the Hertz theory of contact stress. The Hertz theory of contact stress is applicable to the analysis of contact between two fine objects that are surrounded individually by different closed curved surfaces. This theory cannot be complied unless the thickness of each disc is greater enough than the length of the contact surface (elliptic surface). Accordingly, each conventional disc is made thick enough to resist high pressure.
As a demand for the reduction in size and weight of modern continuously variable transmissions of the half-toroidal type increases, however, the section thickness of the discs is expected to be reduced. If the discs are thinned, it may be difficult to use them in compliance with the Hertz theory of contact stress, in some cases. Besides, bending stress acts on the discs. Inevitably, in this state, stress regions near points of contact between the power roller and the discs are disturbed by the bending stress. This results in the production of a greater stress, which may damage the discs. Further, tensile stress that acts on the discs may possibly break the discs.